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طبق خواسته جناب Amirali Fchi ، متن زیر برای ترجمه به ایشان واگذار شد.
خوشه های کروی
Globular clusters (GCs) are aggregates of approximately 104–106 gravitationally bound stars, highly concentrated to the center, spread over a volume ranging from a few dozen up to more than 300 light-years (ly) in diameter.
They resemble shining, old islands orbiting the Milky Way. As the name indicates, GCs show a largely spherical symmetry about their centers. The stellar density in the cluster’s center is so high (up to a few 103 stars ly−3) that it is generally
impossible to separate the individual stars from ground based observations. Only recently has the refurbished Hubble Space Telescope (HST) allowed astronomers to dig
into the very central regions of many Galactic globulars, where members (sometimes peculiar or even exotic) move randomly like molecules of gas, interacting according to
the basic laws of gravity. Early studies of GCs date back to the birth of modern
astronomy. Since then, GCs have continued to offer excitement to both professional astronomers and sky lovers with surprising results, and they constitute a basic
benchmark for our astrophysical understanding.
The MilkyWay hosts about 200 GCs. They form a halo of roughly spherical shape which is highly concentrated around the Galactic center, in the Sagittarius–Scorpius–
Ophiuchus region. The most distant Galactic globulars (such as NGC 2419) are located far beyond the edge of the Galactic disk, at distances out to 300 000 ly.
RADIAL VELOCITY measurements have shown that most of the GCs are orbiting the Galaxy in highly eccentric elliptical orbits, with orbital periods of about
108 yr or even longer. While following their orbits around the Galactic
center, GCs are subject to a variety of perturbations (tidal forces fromthe parent galaxy, passage through the Galactic plane, star escape, internal dynamical evolution, etc)
which make the existing GCs perhaps just the survivors of a much wider population, partially disrupted and spread out throughout the Galactic halo and far beyond. In
this respect, it has been estimated that, within the next ten billion years or so, most of the present Galactic GCs could disappear. On the other hand, we know today
that four clusters in Sagittarius (M54 in particular) are likely members of the Sagittarius Dwarf Elliptical Galaxy (discovered in 1994), currently merging into the central
regions of the MilkyWay.
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